linux-kernel - Re: [PATCH] cpu-topology: warn if NUMA configurations conflicts with lower layer

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Date:   Thu, 2 Jan 2020 11:29:55 +0000
From:   Sudeep Holla <sudeep.holla@....com>
To:     "Zengtao (B)" <prime.zeng@...ilicon.com>
Cc:     Linuxarm <linuxarm@...wei.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [PATCH] cpu-topology: warn if NUMA configurations conflicts with
 lower layer

On Thu, Jan 02, 2020 at 03:05:40AM +0000, Zengtao (B) wrote:
> Hi Sudeep:
>
> Thanks for your reply.
>
> > -----Original Message-----
> > From: Sudeep Holla [mailto:sudeep.holla@....com]
> > Sent: Wednesday, January 01, 2020 12:41 AM
> > To: Zengtao (B)
> > Cc: Linuxarm; Greg Kroah-Hartman; Rafael J. Wysocki;
> > linux-kernel@...r.kernel.org; Sudeep Holla; Morten Rasmussen
> > Subject: Re: [PATCH] cpu-topology: warn if NUMA configurations conflicts
> > with lower layer
> >
> > On Mon, Dec 23, 2019 at 04:16:19PM +0800, z00214469 wrote:
> > > As we know, from sched domain's perspective, the DIE layer should be
> > > larger than or at least equal to the MC layer, and in some cases, MC
> > > is defined by the arch specified hardware, MPIDR for example, but
> > NUMA
> > > can be defined by users,
> >
> > Who are the users you are referring above ?
> For example, when I use QEMU to start a guest linux, I can define the
> NUMA topology of the guest linux whatever i want.

OK and how is the information passed to the kernel ? DT or ACPI ?
We need to fix the miss match if any during the initial parse of those
information.

> > > with the following system configrations:
> >
> > Do you mean ACPI tables or DT or some firmware tables ?
> >
> > > *************************************
> > > NUMA:      	 0-2,  3-7
> >
> > Is the above simply wrong with respect to hardware and it actually match
> > core_siblings ?
> >
> Actually, we can't simply say this is wrong, i just want to show an example.
> And this example also can be:
> NUMA:  0-23,  24-47
> core_siblings:   0-15,  16-31, 32-47
>

Are you sure of the above ? Possible values w.r.t hardware config:
core_siblings:   0-15,  16-23, 24-31, 32-47

But what you have specified above is still wrong core_siblings IMO.


[...]

> > > diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
> > > index 1eb81f11..5fe44b3 100644
> > > --- a/drivers/base/arch_topology.c
> > > +++ b/drivers/base/arch_topology.c
> > > @@ -439,10 +439,18 @@ const struct cpumask
> > *cpu_coregroup_mask(int cpu)
> > >  	if (cpumask_subset(&cpu_topology[cpu].core_sibling, core_mask)) {
> > >  		/* not numa in package, lets use the package siblings */
> > >  		core_mask = &cpu_topology[cpu].core_sibling;
> > > -	}
> > > +	} else
> > > +		pr_warn_once("Warning: suspicous broken topology: cpu:[%d]'s
> > core_sibling:[%*pbl] not a subset of numa node:[%*pbl]\n",
> > > +			cpu, cpumask_pr_args(&cpu_topology[cpu].core_sibling),
> > > +			cpumask_pr_args(core_mask));
> > > +
> >
> > Won't this print warning on all systems that don't have numa within a
> > package ? What are you trying to achieve here ?
>
> Since in my case, when this corner case happens, the linux kernel just fall into
> dead loop with no prompt, here this is a helping message will help a lot.
>

As I said, wrong configurations need to be detected when generating
DT/ACPI if possible. The above will print warning on systems with NUMA
within package.

NUMA:  0-7, 8-15
core_siblings:   0-15

The above is the example where the die has 16 CPUs and 2 NUMA nodes
within a package, your change throws error to the above config which is
wrong.

> >
> > >  	if (cpu_topology[cpu].llc_id != -1) {
> > >  		if (cpumask_subset(&cpu_topology[cpu].llc_sibling, core_mask))
> > >  			core_mask = &cpu_topology[cpu].llc_sibling;
> > > +		else
> > > +			pr_warn_once("Warning: suspicous broken topology:
> > cpu:[%d]'s llc_sibling:[%*pbl] not a subset of numa node:[%*pbl]\n",
> > > +				cpu,
> > cpumask_pr_args(&cpu_topology[cpu].llc_sibling),
> > > +				cpumask_pr_args(core_mask));
> > >  	}
> > >
> >
> > This will trigger warning on all systems that lack cacheinfo topology.
> > I don't understand the intent of this patch at all. Can you explain
> > all the steps you follow and the issue you face ?
>
> Can you show me an example, what I really want to warn is the case that
> NUMA topology conflicts with lower level.
>

I was wrong here, I mis-read this section. I still fail to understand
why the above change is needed. I understood the QEMU example, but you
haven't specified how cacheinfo looks like there.

--
Regards,
Sudeep